1. Field of the Invention
This invention relates to microwave phase shifters and more particularly to a reflection type phase shifter.
2. Description of the Prior Art
One example of a 180.degree. phase shifter is described in a publication entitled "Broad-Band 180.degree. Phase Shift Section in X-Band" by T. Yahara et al., IEEE Trans. Microwave Theory Tech., Vol. MTT-23, pp. 307-309, March, 1975. In FIG. 1 thereof a 3 dB coupler is shown having four ports, one for the input signal, one for the output signal, and two ports which are terminated with PIN diodes. A bias voltage is supplied to the diodes to cause them to be conducting or non-conducting. The phase of the microwave signal at the output port is shifted 180.degree. by switching both diodes from the conducting state to the non-conducting state or vice versa. The impedance of the diode Z.sub.D- for the reverse biased case, and Z.sub.D+ for the forward biased case, have some uncertainties requiring some matching circuits to obtain a specified phase shift and have some differences in frequency dependence which decrease the bandwidth of acceptable phase shifts. A two-stub matching circuit is shown coupled between the anode of each diode and its respective port to provide a wideband reflection type 180.degree. phase shift with a center frequency of 9.5 gigahertz and a bandwidth wider than 2 gigahertz.
In the publication by T. Yahara discussed above a 3 dB interdigitated coupler was shown in FIG. 2 and is of the type described by J. Lange in a publication entitled "Interdigitated Strip Line Quadrature Hybrid", IEEE Trans. Microwave Theory Tech., Vol. MTT-17, pp. 1150-1151, December, 1969. The interdigitated coupler provides wide bandwidth performance with very low insertion loss.
A phase shifter of the reflection type which utilizes a conventional branch line quadrature coupler and PIN diodes is described in U.S. Pat. No. 4,205,282 which issued on May 27, 1980 to J. W. Gipprich entitled "Phase Shifting Circuit Element" which is assigned to the assignee herein. In U.S. Pat. No. 4,205,282, parallel line pairs coupled between each port and the diode determine the amount of phase shift at the output of the coupler for the condition of conducting and non-conducting diodes. DC bias is provided to each diode utilizing a high impedance quarter wavelength line and a low impedance subsection to decouple the bias source from the RF circuitry.
The insertion loss through a phase shifter may vary considerably over the range from 0.degree. to 180.degree. phase shift. The variation in insertion loss results from RF power being dissipated in the PIN diodes at times the diodes are conducting as opposed to times when they are non-conducting. The 3 dB coupler used in the phase shifter is essentially lossless and does not contribute significantly to the insertion loss at either 0.degree. or 180.degree. phase shift. The high speed PIN switching diodes used in phase shifters have a narrow junction resulting in high insertion loss when the diodes are conducting. Compensation techniques using impedance transforming resonant circuits at the diode provide acceptable performance over a narrow band only. In addition, the element values associated with impedance transforming resonant circuits are impractical at lower microwave frequencies such as S-band.
It is therefore desirable to provide a phase shifter having substantially equal insertion loss through the phase shifter for both 0.degree. and 180.degree. phase shift.
It is further desirable to provide a phase shifter having wide band performance over at least an octave bandwidth and high speed switching in less than 10 nanoseconds utilizing a "Lange" 3 dB coupler, PIN diodes and chip resistors coupled in shunt across each switching diode.
It is further desirable to provide a phase shifter incorporating diodes, reactive and resistive elements coupled to the diode to compensate for the reactance and variation in resistance of the diode between the conducting and non-conducting condition.